The present invention relates to improved fluid bed processes in which catalyst losses are significantly reduced. The invention is particularly advantageous in processes for ammoxidation of light hydrocarbons, preferably olefins, to unsaturated nitriles, such as acrylonitrile.
Many processes are known in which a bed of particulate catalyst is fluidized by a gaseous stream containing reactants which, in contact with the catalyst, are converted to desired reaction products. In such processes some catalyst, particularly smaller particle size catalyst, is entrained and must be separated from the product stream after it exits the fluid bed. This is frequently accomplished by cyclone means located downstream of the fluidized bed. Under the action of the cyclone, a major portion of the particulates are separated and recovered while the gaseous product stream passes overhead for further purification, utilization, or packaging. Unfortunately, during the process - most likely under severe conditions in the cyclone - a portion of the catalyst is converted to dust and exits the cyclone with the product stream and is thus considered lost. The problem is particularly apparent in ammoxidation processes where catalysts of relatively small particle sizes are frequently used. Such processes are exemplified, for example, in U.S. Pat. Nos. 3,164,626; 3,335,169; 3,446,834; 3,668,147; and 4,018,712; and 4,590,011; the teachings of which are incorporated herein by reference. U.S. Pat. No. 4,590,011 teaches the admixture of inert particulate materials with active catalyst, such as described above, to improve product yield and inhibit formation of by products.
It has long been apparent that techniques for reducing solids losses in fluid bed processes would represent a significant advance in the art.